acid rain — rain with
an abnormally high acid content caused by burning of
fossil fuels, which release sulfur and nitrogen oxides.
In the presence of sunlight, these pollutants form sulfates
and nitrates that create sulfuric and nitric acids when
dissolved in water.

barometric pressure —
the weight of the atmosphere over a unit area of Earth's
surface. Air pressure fluctuations are associated with
changes in weather.

biome — a major life
zone containing interrelated plants and animals adapted
to a particular climate and soil, which covers a large
geographical area.

CFCs — short for "chlorofluorocarbons,"
a family of industrial gases. CFCs interfere with the
natural cycle of ozone formation in the upper atmosphere,
which acts as a barrier to harmful ultraviolet radiation.

coalescence — process
by which tiny water droplets in a cloud merge to form
raindrops that eventually precipitate.

condensation — formation
of liquid water from vapor, either around a small particle
or on a solid surface. See dew point, evaporation.

convection — transfer
of heat by movement of air or water; specifically, rise
of an air mass caused by warming over land or sea that
often causes the formation of clouds.

cyclone — a rotating
mass of air with minimum pressure in its center. In
the Northern Hemisphere, such winds move counterclockwise,
and in the Southern Hemisphere, clockwise.

dew point — temperature
at which air at constant pressure must cool to become
saturated with water vapor, followed by condensation.

down draft — a rapidly
descending column of cooling air that causes heavy rains
and violent wind gusts.

evaporation — process
by which a liquid such as water becomes a gas.

high pressure system —
an area of high barometric pressure that rotates clockwise
in the Northern Hemisphere and counterclockwise in the
Southern Hemisphere.

hurricane — an extremely
low-pressure, destructive weather cell of tropical origin
with wind speeds in excess of 74 miles per hour (119
kilometers per hour). Hurricanes causes widespread flooding
and wind damage in North America and the Caribbean.

jet stream — fast-moving
currents of air reaching 180 mph (290 kmph ) in the
troposphere — 30,000 to 35,000 feet up (9,144 to
10,668 meters) — that can strengthen and shift
low-pressure systems. In winter, currents may shift
toward the equator; in summer, toward the poles.

low pressure — a system
in which air pressure decreases toward the center, associated
with unsettled weather, usually formed by a mass of
warm air being forced up by cold air.

ozone — a gas composed
of three oxygen atoms that forms a thin diffuse layer
in the upper atmosphere. This layer absorbs harmful
ultraviolet rays from the sun and also prevents heat
loss from Earth.

sublimation — the
change of ice to vapor or vapor to ice without a liquid
phase.

temperature inversion —
an atmospheric condition in which a layer of warm air,
usually from a high pressure system, moves over a colder
layer, preventing it from rising or dispersing. In urban
areas, automobile and industrial air pollutants become
trapped, resulting in a band of smog from the ground
to a height of 500 to 1,000 feet (152 to 305 meters).

typhoon — a term for
a hurricane occurring in the North Pacific or the China
Sea.

wind shear — vertical
eddies or waves of air resulting from one layer of air
sliding over another moving at different speeds or directions.